Coaxial illumination where illuminating light passes through a lens system that also forms the image of the subject is used in examination and viewing equipment. An annular shaped illumination device illuminates a subject while the subject is viewed through the center aperture of the annulus.
The retinal surface of the eye poses a unique challenge in illuminating and imaging. Light must be introduced through the pupil of the eye along the same axis as that of the imaging system. To achieve optimal imaging and viewing of the retinal surface, light must introduced without striking the central anterior structures of the eye, including the cornea and lens. Various means for eliminating reflections from these structures have been employed.
The use of a ring shaped illumination pattern is well known for eliminating reflections from the surfaces of the cornea and lens of the eye. This has been achieved with the use of light projection masks within the illumination and imaging sub-systems. Elimination of reflections from the rear surface of the objective lens has been achieved with beam splitting devices, polarized filters, projection masks and opaque spots applied directly to the rear surface of the objective lens. Although effective, these methods suffer from visible artifacts, light reduction or complex construction. Further, several prior retinal camera and ophthalmoscope designs utilize mirrors or beam splitting devices that must be held in very close proximity to the eye, risking unwanted contact.
Most ophthalmic devices generally utilize separate illumination and imaging paths, placing the illuminating light relatively distant from the objective lens. This arrangement necessitates the use of high wattage filament bulbs for viewing and xenon flash tubes for photographic imaging.
Presently available portable retinal cameras rely on wall current for operation and have not generally been used as examination devices because of their relatively large size.
It is therefore desirable to provide a system and method for reflex-free coaxial illumination that overcomes the limitations, challenges, and obstacles described above.